Polytetrafluoroethylene fabrication with hexafluorobenzene

ABSTRACT

Polytetrafluoroethylene (ptfe) is fabricated into shaped products with the use of hexafluorobenzene as a plasticizer. The ptfe may be particulate, and may be fabricated by ram extrusion. From 8 to 16 percent by volume of the hexafluorobenzene is mixed with the ptfe and maintained in contact with it for long enough to be absorbed onto and just into the surface of the ptfe.

2 United States atent 1151 3,665,067 Hopkm 1451 23, 1972 54] POLYTETRAFLUOROETHYLENE 3,060,517 10/1962 Fields ..264/127 FABRICATION WITH 3,391,221 7/1968 Gore et a1 .264/127 3,528,879 9/1970 Kometani et al... ..264/ I 27 [72] Inventor: William Hopkin, Avonmouth, Bristol, En- OTHER PUBLICATIONS gland Lontz et at, Industrial and Engineering Chemistry, v01. 44, 73 Assignee: Imperial smelting Corporation (N.S.C.) 8,Augl952mases 3 1 Limited, London, England Mellan, Source Book of Industrial Solvents, Vol. 11

d Filed: J y 1970 (Halogenated Hy rocarbons), Reinhold Publishing Co New [21] Appl. No.: 64,062

Related US. Application Data [63] Continuation of Ser. No. 610,192, Jan. 18, 1967, abandoned.

[52] US. Cl ..264/127, 260/33.8 F [51] Int. Cl ..B29f 3/00, C081 45/30 [58] Field ofSearch ..264/127; 260/33.8F

[56] References Cited UNITED STATES PATENTS 2,510,112 6/1950 I-Iolbrook ..260/33.8F

York, 1957, p. 43.

Primary Examiner-Allan Lieberman Attorney-Lane, Aitken, Dunner & Ziems [5 7] ABSTRACT Polytetrafluoroethylene (ptfe) is fabricated into shaped products with the use of hexafluorobenzene as a plasticizer. The ptfe may be particulate, and may be fabricated by ram extrusion. From 8 to 16 percent by volume of the hexafluorobenzene is mixed with the ptfe and maintained in contact with it for long enough to be absorbed onto and just into the surface of the ptfe.

7 Claim, 33 Drawing Figures POLYTETRAFLUOROETHYLENE FABRICATION WITH HEXAFLUOROBENZENE This application is a continuation of application Ser. No. 610,192, filed Jan. 18, 1967 and now abandoned.

This invention relates to the fabrication of shaped articles from thermoplastic polymers.

In the fabrication of high melting thermoplastic polymers, it is frequently necessary to resort to the techniques of powder metallurgy in order to compact the moulding powder to a useful form. This is particularly necessary in the case of polytetrafluoroethylene. In this case, compacting by the use of high pressure in the cold compresses individual particles and closes up internal voids, and ensures large areas of contact between contiguous neighboring particles in the matrix. Subsequently, treatment at a temperature above the melting points of the crystallites, eg in the region of 380 C, enables a limited amount of flow to take place (sintering) whereby the particles fuse together at the areas of contact.

However, two troublesome features of these operations are that complete expulsion of gases, and thus complete filling of the interstices of polymer during the cold compacting of granules is difficult, and voids are frequently observed after sintering; further high moulding pressures are required even to achieve these marginal improvements. Thus much effort has been expended in the development of polymer particles which minimises void formation.

The most frequent way in which this is carried out is by preparing polymer of finer particle size, but this makes the problem of flow of the raw polymer more and more difficult. The most satisfactory method for void-free polymer uses the so called paste extrusion process, but this is unsatisfactory as a process for producing shaped articles or mouldings. Further, this fine polymer is considerably more expensive than the granular grades.

The vital step in cold compacting is the compaction or the consolidation of the individual particles. If the particles, or even their surfaces were softer, more flow could take place, greater contact areas would result, and mouldings more free of voids and flaws would result. This condition is normally achieved by swelling the polymer with some volatile solvent. But cold compacting is also markedly improved if the polymer particles can be lubricated so that they flow under the pressure of the compacting device more easily and thus fill out the fixed dimensions of the compacting mould.

The present invention provides a method for the fabrication of polytetrafluoroethylene (referred to hereafter as ptfe) into a shaped product wherein it is processed in the presence of hexafluorobenzene.

The nature of such fabrication will generally be that granules or particles of ptfe are contacted with hexafiuorobenzene in such quantity as to leave some hexafluorobenzene upon the surface of the granules or particles of ptfe, the particles or granules thereafter being subjected to some form of compaction, followed by heating to a sintering temperature.

The compaction is generally effected in a mould, but may also be carried out by ram extrusion, including thin-section extrusion. The presence of hexafluorobenzene with the ptfe greatly aids the extrusion of thin sections.

The present invention also provides a method of fabricating shaped products of ptfe, which method comprises mixing from 5 to percent by volume, on the volume of the ptfe, of hexafiuorobenzene with the ptfe, and then applying pressure to the ptfe to fabricate it into the desired shapes.

Fabrication" in the preceding paragraphs includes compaction, prior to sintering. It also includes welding of ptfe, in which process the surfaces to be welded are soaked or wetted with hexafluorobenzene, and are then welded by the application of heat and light pressure. Alternatively, a previously soaked thin strip may be interposed between the faces of a butt type weld. Fabrication also includes extrusion, whether ram extrusion or thin-section extrusion, for e.g. thinwall tubing or wire sleeving. The invention is particularly advantageous when ram extrusion is performed, particularly when pressures below 5,000 psi are employed.

It is preferred to employ from 8 to 16 percent by volume, on the volume of the ptfe, of hexafluorobenzene. Proportions of hexafluorobenzene above 16 percent can be used, but the high cost of hexafluorobenzene tends to make the process less economical. For this reason, it is desirable to use the minimum, or at least a level approaching the minimum, effective amount of hexafluorobenzene. In ram extrusion equi ment, or repetitive injection moulding equipment, a ram travels up and down as a piston, and batches of polymer granules are introduced from a hopper into the cylinder near the end of each cycle. Thus there is a measurable rate of flow of polymer through the machine, and thus a residence time" of material in the hopper can be determined; clearly it will depend on whether batch-wise or continuous feed to the hopper is used. Thus, where continuous feed is in use, it is possible to mix the ptfe and hexafluorobenzene in the hopper, and, by careful control, maintain the residence time of the mixture in the hopper approximately constant.

For maximum economy of hexafluorobenzene the residence time under such flow conditions is important. We have surprisingly found that hexafiuorobenzene is one of the very few organic solvents which will swell ptfe, either as a shaped sintered product or as raw polymer granules. But a period of time is needed to achieve equilibrium swelling of ptfe by hexafluorobenzene, which time period is determined by a variety of factors, the dimensions and surface area of the polymer particles being important ones. However, ptfe which has been allowed to swell to equilibrium with hexafluorobenzene, and which contains no excess hexafluorobenzene, shows no improvement in extrusion performance. Thus if it is desired to use less than the equilibrium amount, the residence time in constant flow equipment must be adjusted to be less than the time required to reach equilibrium swelling. Thus when 5 percent hexafluorobenzene, by volume on the volume of ptfe, is used a residence time of less than 1 hour, preferably in the region of 15 minutes will be necessary; where 10 percent of hexafluorobenzene, by volume on the volume of ptfe, is used a residence time of less than 6 hours, preferably in the range 1% to 3 hours, is necessary. However, since no two machines perform the same, and no two samples of polymer have the same characteristics, the precise volume percentage required for a given residence time can only be determined by experiment.

In the case of batch-operated equipment the quantity of hexafluorobenzene, by volume on the volume of ptfe, will have to be a little higher than the equilibrium amount in order to over come the lack of improvement found under conditions where all the hexafluorobenzene is absorbed into the body of polymer granules.

It will be appreciated that this process is applicable to the various grades of ptfe polymer available, that is to say, to the normal grade to fine grade and the ultra-fine grade. These particles or granules are more or less spherical or else are agglomerates of spheres.

Since the swollen and softened granules are fairly stable on storage, and may possibly in themselves figure as an article of commerce, it will be appreciated that granules or particles of ptfe mixed with hexafluorobenzene constitute another aspect of the present invention.

On sintering the hexafluorobenzene is volatalized and ptfe articles substantially identical with the conventionally prepared articles, with the exception of lower void content result, as may be shown by conventional physical testing and dye penetrant tests. (Thick section articles require special treatment in order to remove all the hexafluorobenzene; ageing at a more or less elevated temperature below the sintering point, e.g. about 300C, with or without the use of a vacuum is effective.)

As indicated above, this process is not confined to moulding, since the essential feature of softening the surface of a polymer particle is capable of extension to all particle fabrication methods.

Thus, ram extrusion is made more effective. Ram extrusion takes polymer granules from a feed hopper which granules or particles are then compacted or consolidated by a ram.

Withdrawal of the ram allows more polymer granules to be charged on top of the largely consolidated billet for subsequent consolidation. The major weakness of this process lies in the absolute necessity for perfect welding of contiguous against the shoulders 13 of the body 10 of the die. The hole 12 was loosely packed with moulding material (approximately 1 1 gm of unadulterated ptfe filled the mould) up to the level of the top 18 of the body 10. The base plug 16 was unscrewed to granule charges. Failure to ensure this results in transverse lea e a a f(),050 in, between the aluminum disc 15 and the cracks and weaknesses. The use of hexafluorobenzene enables shoulders 13; with the ZOTPI thread used, this corresponds to this welding of successive charges to take place much more backing off by one turn. The body was then inserted into readily, and substantially perfect billets are obtained with the sleeve 11, and the piston 17 put into place. comparative 'm m m? extrusion -8- 0f 10 The assembled mould was placed in a hydraulic press, and wall tubmg wll'e coatings: also faclhtatefithe ram adjusted to zero pressure. The pressure was then built Compaction y be camed out substanzlany normal up across the faces 19 (top of the piston) and 20 (bottom of temperatures f emblem *9 about 300 s eme the base plug) uniformly to that desired 2,500, 5,000, or tures can be disadvantageous in that premature volatilization 7500 over 1 minute, and maintained m at that pressure of the hexafluol'obenzene y resultfor 3 minutes. It was then released as quickly as the press al- Recovery. of hexaflflol'obenzene highly desirable lowed (less than 5 seconds). The die was then removed from tfecause of high cost; may be camedput by com/en the press, dismantled and the top hat" removed (the extru- Onal Solvent 'y Processes, -gp p f followed sions are coherent enough to stand up to gentle handling). by refrigeration or absorption on charcoal and the like.

Preparation of the moulding materials was effected as fol- Conventional materials are also known which have the same lows. The ptfe powder was rubbed through a l0 mesh sieve effect of improving the quality of ptfe mouldings or extrusions.

and the required amount weighed into a 4 oz. screw cap bot- But these all suffer from the disadvantage that unlike hextle, together with the required amount of additive. The bottle afluorobenzene they are either not easily removable, or else was then rotated at 200 rpm. approximately for the desired not removable at all from the extruded matrix prior to sintertime. In control experiments the powder was merely rubbed mg e,g. graphite and mineral oil. The presence of such materithrough the Sieve before Us: als in the extruded product renders the sintered product useless since they degrade under the sintering conditions. The loadmg of addmve m an cases a f on the use f 1 The invention is further described with reference to the acvolume of Ptfe Powder 1 volumfis of addmvai thus a loadmg companying drawings in which: of 10 percent hexafluorobenzene is 10 ml. solid ptfe plus 1 ml FIG. 1 is a sectional side elevation of an extrusion die, and of haxafluofobenzene- The we'ght of P requu'ed 15 based on FIG. 2A-2L to 6A-6C each show ptfe samples (in plan and a Sohd dawn) of gm/cc' i extruded y m FIGS. 2 to 5 shows the extrusions obtained from five series Refemng to 1, the (he Compnses a body 10 m asleeve of experiments. In assessing the results, the two important 11, the body having a cylindrical hole at 12 to contain ptfe parameters are granules, which hole is of enlarged diameter at its lower end a, how much f the powder has been extruded f the below downwardly facing shoulders 13. The enlarged portion b d f h h i h b i as i di ated by the of the hole is screw threaded at 14. An aluminum disc 15, hi k f h maining body polished on its upper surface, bears against the shoulders 13, b, h f th owd ha been extruded from the body of and is supported on a base plug 16 which is threaded at 14 and th hat int th b -1 screws into the enlarged portion of the hole. A piston 17 fits 40 It h ld b borne i i d th these experiments were into the sleeve 11 and the Cylindri l hol at 12 h a formed under very stringent conditions (0.050 in. is a narrow clearance of from 0.003 to 0.005 inch. A 20TPI screw thread opening and 2,500 psi not a high pressure); the remark is used at 14. satisfactory" indicates that good results would be obtained In the threaded base p g 16 was Screwed fully home under these conditions of loading and contact time in comagainst the aluminum disc 15, which in turn was pushed home mercial practice.

Proporti'on, Mixing Pressure, Figure Additive percent conditions p.s.i. Remarks 2, 500 Bad. 5,000 Satisfactory. 1,500 D0. 2,500 Do. 5,000 Do. 7,500 Do. 2, 500 Bad. 5, 000 Satisfactory. 7, 500 D0. 2, 500 Poor. 5,000 Satisfactory.

, 500 D0. 2, 500 Bad. 2,500 Satisfactory. 2, 600 D0. 2,500 Bad. 2, 500 Bad. 2, 500 Satisfactory. 2, 500 D0. 2,500 Bad. 2, 500 D0. 2,500 Satisfactory. 2, 500 Bad. 2 hr. tumble 2, 600 Satisfactory. C l,1,1-trichlor0ctliand 10 do r 2,500 Bad. I) Mineral oil 10 d0 2,500 Do. E Molybdenum 10 ...do 2,500 Do.

disulphide. F Water 10 do 2,500 Do. G 4., Graphite r. 10 .d0 600 Do.

Hexafluorobenzcnc 10 10min. tumble. 2,500 Satisfactory. Ila .do 10 10 min. tumble plus 24 hrs. 2, 500 Bail. standing. C ,d0... l0 24lirs tumbliih." 2, 500 no.

I lly volume on voltiinv 0f 1M0.

Determination of the Volume Equilibrium Swelling of Polytetrafluoroethylene by hexafluorobenzene.

This was carried out by two different methods on two separate samples of sintered ptfe sheet.

A. By observing the increase in volume of sample in contact with excess hexafluorobenzene.

Weights: dry sample 0.8935 gm.

swollen sample 0.9685 gm Volume: dry sample 0.4194 cc. swollen sample 0.4641 cc.

by displacement ofwater in a specific gravity bottle. Hence volume swell: 10.6%

B. By observing the length only of a strip immersed in excess hexafluorobenzene.

This method assumes that the product is anisotropicwhich a sintered product rarely is; two strips were cut from the same sample.

Initial lengths: 7.2l5 cm, 7.148 cm Swollen lengths: 7.375 cm, 7.323 cm Equilibrium volume swellings: 6.80%, 7.52%.

ticles, wherein said hexafluorobenzene is maintained in contact with said particles for such time as to ensure absorbtion of the hexafluorobenzene by the polytetrafluoroethylene to bring about swelling of the particles and to also ensure the presence of an excess lubricating amount of hexafluorobenzene, and applying pressure to the polytetrafluoroethylene particles to fabricate them in the desired shape.

2. A method as claimed in claim 1 wherein the hexafluorobenzene and the polytetrafluoroethylene are maintained in contact for a period of time not exceeding 1 hour when 5 percent of hexafluorobenzene is used and not exceeding 24 hours when 15 percent of hexafluorobenzene is used, the maximum contact time for intermediate proportions of hexafluorobenzene being intermediate the periods stated.

3. A method as claimed 1 wherein the range of hexafluorobenzene is from 8 to 16 percent by volume as compared to the volume of the polytetrafluoroethylene.

4. A method as claimed in claim 1 wherein the fabrication is performed by ram extrusion.

5. A method as claimed in claim 4 wherein the extrusion is performed at a pressure of not more than 5 ,000 psi.

6. A method as claimed in claim 5, wherein the extrusion is performed at a pressure of from 2,000 to 3,000 psi.

7. Finely divided granules or particles of polytetrafluoroethylene softened with and swollen by 5 to 20 percent volume of the polytetrafluoroethylene of hexafluorobenzene, said hexafluorobenzene being present in an amount in excess of that which is absorbed by said polytetrafluoroethylene in order to bring about said swelling. 

2. A method as claimed in claim 1 wherein the hexafluorobenzene and the polytetrafluoroethylene are maintained in contact for a period of time not exceeding 1 hour when 5 percent of hexafluorobenzene is used and not exceeding 24 hours when 15 percent of hexafluorobenzene is used, the maximum contact time for intermediate proportions of hexafluorobenzene being intermediate the periods stated.
 3. A method as claimed 1 wherein the range of hexafluorobenzene is from 8 to 16 percent by volume as compared to the volume of the polytetrafluoroethylene.
 4. A method as claimed in claim 1 wherein the fabrication is performed by ram extrusion.
 5. A method as claimed in claim 4 wherein the extrusion is performed at a pressure of not more than 5,000 psi.
 6. A method as claimed in claim 5, wherein the extrusion is performed at a pressure of from 2,000 to 3,000 psi.
 7. Finely divided granules or particles of polytetrafluoroethylene softened with and swollen by 5 to 20 percent volume of the polytetrafluoroethylene of hexafluorobenzene, said hexafluorobenzene being present in an amount in excess of that which is absorbed by said polytetrafluoroethylene in order to bring about said swelling. 